As optical fiber progressively supplements and replaces metal wire as the backbone of telecommunications networks, the switches that route optical signals have emerged as a significant bottleneck. Transmission systems move information as optical photons but the switching systems and so-called crossconnect fabrics that switch, route, multiplex, and demultiplex optical signals have generally been electronic. Electronic switching requires light to be converted to an electronic signal to pass through the switch and then be reconverted to light in a process termed optical-electronic-optical (OEO) conversion that introduces both time delay and cost.
There is great interest in the telecommunications industry, therefore, in developing all optical switching to avoid the necessity of multiple OEO conversions. On long haul networks, ten's or hundred's of individual wavelengths, each carrying a signal, are multiplexed onto each fiber. Switches are desired that provide all optical switching at the fiber level, the wavelength level, or at both levels. As described, for example, by Bishop et al. in Scientific American (January, 2001, pp 88-94), all optical switches based. on a number of underlying technologies including Micro Electro Mechanical Systems (MEMS) tilting mirrors, thermo-optical devices, bubbles formed by inkjet printing heads, and liquid crystals, have been proposed. Optical fiber switches based on MEMS mirrors are particularly attractive because they can incorporate very large scale integrated circuits and can be robust, long-lived, and scalable.
An optical fiber switch described in U.S. Pat. No. 5,960,132 to Lin, for example, includes an array of hinged MEMS mirrors, each of which can be rotated about its hinge between a reflective state and a non-reflective state. An array of N2 such mirrors is required to switch signals carried by N input optical fibers from one to another of N output optical fibers. Unfortunately, N2 scaling results in unmanageably complex devices for large N.
Another optical fiber switch described in Bishop et al., cited above, as well as in Bishop et al., Photonics Spectra (March 2000, pp. 167-169) includes an array of MEMS mirrors disposed on a single surface. Each mirror tilts independently to direct light received from an array of input/output optical fibers to any other mirror and thus to any input/output fiber. No internal optical diagnostics for this switch have been described in publications to date.
Still other optical fiber switches are based on two arrays of MEMS mirrors that can be tilted in any direction. Incoming light is directed onto a mirror in the first array which deflects it onto a predetermined mirror in the second array. The mirror in the second array, in turn, directs the light received at a particular input mirror to the predetermined output port. In these so-called, 2N configurations, the position of the mirrors has to be controlled very precisely, to small fractions of degrees, to provide the desired connections.
The precise positioning of mirrors can be affected by environmental factors, such as vibration, and changes in temperature or humidity, and by slow drift of voltages used to control the mirrors. These ongoing effects on the alignment and control of mirrors causes difficulty and degradation in the performance of optical switches based on MEMS mirrors.
Therefore, there is a need for improved methods of calibration and control of optical switches in order to further the development of fiber optic telecommunication networks.